1. Field of the Invention
The present invention relates to a method of correcting thermal displacement in a machine tool.
2. Description of the Related Art
In a machine tool, due to heat generated from a motor which drives a feed screw and a spindle, frictional heat caused by rotation of a bearing, and frictional heat at the joined portion of a ball screw and a ball nut, the feed screw and the spindle expand and the tool position is displaced. Namely, the relative positional relationship between the workpiece which is to be positioned and the tool becomes offset. This displacement of the tool position due to heat is problematic in cases where highly precise machining is to be carried out.
As means for eliminating displacement of a tool position due to heat, conventionally, methods have been used in which a cooling apparatus is provided, or an initial tension is imparted to the ball nut at the feed screw so as not to be influenced by an expansion due to heat, or in which a displacement sensor or a temperature sensor is provided and an instructed position is corrected based on the detected displacement or the detected temperature.
However, in the method in which an initial tension is imparted to the ball screw at the feed screw so as not to be influenced by an expansion due to heat and in the method in which a sensor is provided and the thermal displacement is corrected, there are problems concerning limits to the structure or position where a sensor is mounted, or problems in that means is needed to protect the sensor from coolant and chips for maintaining reliability of the sensor. Further, in a case where a sensor is provided, problems arise in that time is required for measurement, and as a result, the machining time increases, and displacement cannot be corrected at the time of start-up of the machining operation.
An object of the present invention is to provide a method of correcting thermal displacement in which thermal displacement can be corrected easily and at a low cost without the need for a sensor.
In a first embodiment of the present invention, in a machine tool which is controlled by using a numerical control device, a correction amount is determined for thermal displacement due to heat generated in a spindle motor and resultant heat conduction, in addition to thermal displacement due to heat generated by the rotation of a spindle. The tool position is corrected based on this correction amount.
The correcting method of the first embodiment comprises: a step of detecting a spindle speed and a load of a spindle motor; a step of determining an average spindle speed and an average spindle motor load based on the spindle speed and the load of the spindle motor; a step of determining an amount of thermal displacement of a spindle portion based on the average spindle speed and the average spindle motor load; and a step of, by using an amount for canceling the amount of thermal displacement of the spindle portion as correction amount, correcting a position instruction value of a feed shaft of the machine tool based on the correction amount.
As a method of determining a thermal displacement due to heat generation of the spindle and heat generation of the spindle motor, a computational formula for computing the thermal displacement from the rotational speed of the spindle and the load of the spindle motor is determined in advance. To correct the tool position at the time of actually driving the machine tool, the rotational speed of the spindle and the load of the spindle motor are monitored, and the monitored rotational speed of the spindle and load of the spindle motor are substituted into the computational formula, and a thermal displacement of a spindle and a thermal displacement due to heat generation of a spindle motor are determined, respectively. The determined thermal displacement of a spindle and thermal displacement due to heat generation of a spindle motor are added, and the sum is used as a spindle portion thermal displacement. An amount which cancels this spindle portion thermal displacement is determined as the correction amount, and the tool position is corrected based on the determined correction amount.
According to the above-described method of correcting thermal displacement of the first embodiment, not only the heat generated by the spindle rotating, but also, the heat generated by the spindle motor is considered as a cause of thermal displacement. Thus, even in a case where there is a high frequency of accelerating and decelerating such as in tapping and the amount of heat generated by the spindle motor is large, the thermal displacement of the spindle portion can be corrected accurately. Further, the spindle portion thermal displacement can be made to include not only the thermal displacement due to the amount of heat generated from the spindle and the spindle motor, but also, the thermal displacement due to the heat radiation and heat conduction caused in the tool portion.
In a second embodiment of the present invention, in a machine tool which is controlled by using a numerical control device, the entire stroke of the feed shaft is divided into a finite number of sections, and the thermal displacement of each section is used. For each of the sections obtained by dividing the entire stroke of the feed shaft into a finite number of sections, a correction amount for the thermal displacement due to axial movement of the feed shaft is determined, and based on the determined correction amount, the tool position is corrected.
The correction method of the second embodiment comprises: a step of dividing a range of movement of a feed shaft to set a plurality of sections; a step of detecting a position of the feed shaft; a step of determining an average moving speed at each section based on the detected position; a step of determining a thermal displacement at each section based on the average moving speed for each section; a step of adding the thermal displacement of each section from a reference position to a correction position, and using the sum as a feed shaft portion thermal displacement; a step of using the sum, which is obtained by adding the spindle portion thermal displacement to the feed shaft portion thermal displacement, as an overall thermal displacement at the correction position; and a step of using an amount, which cancels the overall thermal displacement, as a correction amount, and, in movement to the correction position of the feed shaft, effecting correction by adding the correction amount to a position instruction value of the feed shaft.
As a method of determining thermal displacement due to the feed shaft, a feed shaft displacement computational formula, whose parameter is the moving speed at each divisional section, is prepared.
To correct the tool position at the time of actually driving the machine tool, the position of the feed shaft is monitored, and the average moving speed of the feed shaft at each section from the monitored position is determined of each section. By substituting the determined average moving speed into the feed shaft thermal displacement computational formula, the thermal displacement is determined. By adding the thermal displacement of each section from the reference point to the correction position, the correction amount at the correction position is determined. Based on the determined correction amount, the tool position is corrected. Further, the computational formula may be a formula which includes thermal displacement due to heat radiation at each section and heat conduction from the adjacent section.
According to the above-described method of correcting thermal displacement of the second embodiment, for each of divisional sections obtained by dividing the entire stroke of the feed shaft, the effect of the generated heat at the section to which the feed screw has actually moved, and the effect of the generated heat transmitted by thermal conduction from the adjacent section, are estimated for each section. The thermal displacement for each section is added from the reference point to the correction position. Thus, accurate correction which does not depend on the position can be carried out. Further, by considering the effects of the spindle and the feed shaft, accurate correction can be carried out.
In a third embodiment of the present invention, in a machine tool which is controlled by using a numerical control device, in addition to the thermal displacement of the feed shaft, the thermal displacement due to the feed shaft motor is also considered. The feed shaft portion thermal displacement includes the displacement due to heat generation of the feed screw portion and the thermal displacement due to heat generated in the feed shaft motor. A correction amount for both thermal displacements is computed, and the position of the tool is corrected based on the correction amount.
According to the thermal displacement correction method of the third embodiment, in addition to the effects of heat generated in the feed shaft, the effects of heat generated in the feed shaft motor are considered, and accurate correction can be carried out.
In a fourth embodiment of the present invention, the results of correction are fed-back, and the thermal displacement computational formula is corrected. By using the thermal displacement determined in any of the above-described first through third embodiments and the displacement (correction error) determined from a deviation from an actual tool position, the heat generation factor included in the thermal displacement computational formula is corrected, and the correction error is corrected.
According to the thermal displacement correction method of the fourth embodiment, accurate correction can be carried out based on the actual driving state.
According to the method of correcting thermal displacement of the present invention, thermal displacement can be corrected simply and at a low cost, without the need for a sensor.